Conventionally, structure of cargo tanks of any conventional liquid cargo tanker having known double-hull structure disposes a U-shaped internal hull plating inside of a U-shaped external side plating across a predetermined distance between them.
Internal space generated by provision of the U-shaped internal hull plating in a tanker is made available for a plurality of cargo tanks for storing liquid cargo, whereas another U-shaped space domain between the internal and external hull platings is made available for a plurality of ballast tanks.
Conventional cargo tanks formed inside of the U-shaped internal side plating of any conventional tanker are respectively sectioned into a center tank and wing tanks by a pair of longitudinal bulkheads. In addition, these cargo tanks are also sectioned into a plurality of tanks along cargo tank length fore and aft by means of a plurality of transverse bulkheads. In the same way, the above-cited conventional ballast tanks are also sectioned into a plurality of tanks along cargo tank length fore and aft by means of a plurality of transverse bulkheads.
However, according to the above-cited conventional hull structure, in regard to the U-shaped space domain available for making up the above-cited ballast tanks, in order to secure absolute volume of ballast and absolute strength of the hull structure, normally, depth (width) provided for the bottom and side construction of the hull exceeds the actually required value.
Furthermore, since any conventional tanker utilizes U-shaped space domain between the internal and external hull platings of the hull for making up ballast tanks, when the tanker is fully loaded with cargo, all the cargo tanks are filled with liquid cargo, thus causing extraordinary longitudinal bending moment to adversely affect the hull structure. This in turn necessitates shipbuilders to provide the hull structure with large-scale reinforcement. Furthermore, even when effectuating reinforcement, if the allowance against the reinforced hull strength were too short, then, degree of freedom for cargo loading will disadvantageously be constrained.